Austenitic steels are normally used for manufacturing components of the exhaust system that are subjected to high thermal loads. Furthermore, it has been demonstrated in exhaust treatment devices where at least one exhaust treatment insert, particularly a catalytic converter element, is arranged in a housing, that it is advantageous for the mounting of the exhaust treatment insert, to manufacture the housing from a ferritic steel. Accordingly mixed ferritic-austenitic connections must necessarily be used inside an exhaust system that consists of ferritic components and austenitic components.
In modern internal combustion engines such as those that are used in motor vehicles in particular, increasing exhaust temperatures are being observed, which may result in failures due to thermal fatigue in the area of the mixed ferrite-austenitic connections mentioned. To avoid these thermally conditioned fatigue phenomena it is possible, in principle, to construct the area of the exhaust system at risk purely from ferrite or purely from austenite. A purely austenitic or purely ferritic construction of the thermally loaded areas of the exhaust system is not always possible, however, because ferritic materials have insufficient temperature resistance for many applications, for example. At the same time austenitic materials may be unsuitable for mounting an exhaust treatment insert, e.g. a catalytic converter.
One aspect of the present invention is concerned with the problem of proposing an improved design for an exhaust system or an exhaust treatment device of the type already mentioned in which, in particular, the risk of thermally conditioned fatigue phenomena is reduced, even though both austenitic and ferritic materials are used in construction.